An experimental study of the optimum position of the drainage gallery beneath concrete gravity dam

A gravity dam relies primarily on its self-weight for stability, making the control of uplift pressure beneath the foundation essential. This study experimentally investigates the optimal horizontal and vertical placement of a drainage gallery using a Hele-Shaw model constructed in the Irrigation and Hydraulics Department laboratory at Alexandria University. The relative horizontal position of the gallery is expressed as (x/B), where (x) is the distance from the dam heel and (B) is the base width, while the relative depth is expressed as (d/H), where d is the gallery depth and (H) is the upstream water height. The investigation assumes a steady-state seepage condition through a simplified, homogeneous, and isotropic foundation material beneath the dam. The values of parameter (x/B) in the experiment are 0.11, 0.28, 0.44, 0.61, and 0.78 respectively and the values of parameter (d/H) are 0.026, 0.061, 0.096, 0.130, and 0.165 respectively. Fluid viscosity and hydraulic conductivity were dynamically calculated based on oil temperature variations. Results show that the most effective configuration which minimize the uplift pressure (maximum reduction in uplift pressure force) and maximize the seepage quantity passing through the drainage gallery occurs at (x/B) = 0.15 and (d/H) = 0.165, achieving nearly 70% reduction in uplift pressure. Also, increasing the depth of the drainage gallery (d/H) increases the seepage quantity and decreases the uplift pressure force. In comparison, the theoretical formulation of¹¹ indicates an optimum at (x/B) = 0.125, with a predicted reduction of approximately 56%. Additionally, increasing the horizontal distance of the gallery reduces seepage discharge and increases the uplift pressure. These experimental findings offer refined geometric guidelines for gravity dam foundation design, allowing engineers to enhance structural safety margins against sliding, optimize foundation drainage layouts, and reduce excessive conservative safety factors in hydraulic infrastructure projects. Also, these findings provide preliminary experimental guidance that should be validated for different geometries and site conditions.